What is the River Discharge and
What Factors Affect it?
Drainage Basin
This is the area of land drained by a river system (a river and
its tributaries).
The drainage basin system is said to be open as
both inputs and outputs of energy and material
occur. All rivers receive a water from it. The
boundaries of the basin are known as
the watershed and will usually be marked by
areas of higher land. Drainage basins have many
different characteristics that influence how
quickly or slowly the main river within them
responds to a period of intense rainfall, these are
outlined in more detail in the section relating
to storm hydrographs.
River Discharge
The discharge of a river is the volume of water which
flows through it in a given time. It is usually measured in
cubic meters per second.
Calculation:
Cross-sectional area of channel (m2)
X
Velocity of the river / water (m/s)
*This gives discharge as the volume (m3/s) or cumecs.
How is river discharge affected by
physical factors?
• Impermeable Rock (e.g. granite) – Water is unable to infiltrate
through, resulting in more surface runoff, increasing volume of the
channel and its speed.
• Permeable Rock – More infiltration, resulting in less surface runoff
and less volume in the river
• Size of drainage basin – Small  Water will enter the river quicker
and faster
• Relief of drainage basin – If the slope of the basin is more steep,
water in the river is likely to move down faster, increasing its speed
• Percipitation – heavy rain can cause saturation in the soil and hence
cause more water to reach the river (runoff). This also means that
the speed of the river increases.
• Vegetation – allows more infiltration and interception, causing less
surface-runoff and slowing down the speed of the river
How is river discharge affected by
human factors?
• Impermeable man-made surfaces – Concrete and
tarmac can cause rivers in urban drainage basins to
have a higher discharge due to higher amounts of
surface runoff. Speed is also increased due to drainage
systems and ground.
• Destruction of vegetation (deforestation) – Less
infiltration + interception causes more surface run off
and increases speed of the water.
• River Management – Presence of dams allow river flow
to be controlled, which may cause more discharge
(before the dam) , or less (below the dam).
Hydrographs
Graphs which shows how the river discharge in a
drainage basin responds to a period of rain.
Base flow – the normal day to
day discharge of a river
The rising limb – the rapid increase
of discharge resulting from a
rainfall.
Peak flow – when the river reaches
the maximum capacity that it can
hold.
The recession limb – when the
discharge starts to decrease and
river levels fall.
Basin lag time – the time difference
between the peak of the rain event
to the peak flow.
What factors affect the shape of flood
hydrographs?
Drainage basin
- Type of rock (impermeable or permeable) –
Impermeable rock will not allow water to seep in, thus
causing larger amounts of surface runoff and a shorter
lag time.
- The gradient of the drainage basin – Steep gradients
will cause greater overland flow and a shorter lag time.
- Size of drainage basin – larger basins will take longer
to reach the river, hence a longer lag time
- Present conditions of the drainage basin – soil either
saturated, very dry or even frozen
Type and amount of Precipitation
- Rapid rain – soil will saturate at a very rapid rate,
excess water quickly transfers by surface runoff thus
causing a short lag time
Land Use and Human Impact
- Impermeable man-made surfaces – e.g. concrete
and tarmac roads, shorter lag times
- Vegetation area –infiltrates more and intercepts
water, a longer lag time, reducing discharge
- Area of deforestation – short lag time, increases
discharge
Time/season of the year
- Summer – evapotranspiration rates are higher,
reducing surface run off, longer lag time
- Temperatures
Water Use
- Dams and reservoirs near area – slow down
the rate of discharge, a much longer lag time,
and may also cause a reduced amount of
discharge
How do flood hydrographs change as
river discharge changes?
• When it starts to precipitate, the level of discharge starts fairly low
at base flow level (1).
• As the rain starts to get heavy, the soil may be saturated, thus there
is an increase in surface run off. This causes water to start flowing
into the river channel, causing the level of discharge to increase,
making the line go up positively.
• A very steep rising limb (2) would indicate a fast increasing
discharge rate meaning water flowing into the river channel is a
faster rate, and thus a heavier rainfall.
• (3) When the amount of water in the river is at its maximum level
(peak discharge), the curve will reach its highest point on the graph.
• As river discharge starts to decrease (4), the curve will start to slop
gradually (gradient is normally less steep compared to the lag time).